This invention relates to light emitting devices and in particular to light emitting diodes. More particularly, this invention concerns light emitting devices which use the emission from an electrically powered light emitting semiconductor device to stimulate photoluminescence in a material adjacent the semiconductor device.
Solid state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. Photonic devices are often classified into three categories: light sources (light emitting diodes, lasers, diode lasers, etc.), photodetectors (photoconductors, photodiodes, etc.), and energy conversion devices (photovoltaic cells).
More specifically, light emitting diodes (LEDs) are semiconducting devices that emit light (including ultraviolet, visible, and infrared light) when a potential difference is applied across a p-n junction structure. There are a number of ways to make light emitting diodes and many associated structures, but these are generally well known, and the invention that will be described herein applies to most or all of them. Thus, they will not be discussed in detail hereinafter except as necessary to explain the invention. By way of example, and not of limitation, Chapters 12–14 of Sze, Physics of Semiconductor Devices, (2d Ed. 1981) and Chapter 7 of Sze, Modern Semiconductor Device Physics (1998) give a good explanation of a variety of photonic devices, including LEDs.
The term LED is used herein to refer to the basic semiconductor diode structure. The commonly recognized and commercially available “light emitting diode” that is sold (for example) in electronics stores typically represents a “packaged” device made up of numerous parts. These packaged devices typically include a semiconductor based LED such as (but not limited to) those described in U.S. Pat. Nos. 4,918,487; 5,631,190; and 5,912,477; various wire connections, and a package that encapsulates and protects the LED.
In many commercial embodiments, the package comprises a hard polymeric encapsulant material, which supplies a high degree of mechanical stability to the device. The package may also provide desirable optical characteristics such as color, shape (i.e., lensing) and refractive index. Various polymers have been used successfully over the years by different manufacturers as encapsulant material. Suitable polymers used in the past include cycloalaphatics, polymethyl methacrylate (PMMA), epoxies and polyurethanes.
Although the development of the LED has in many ways revolutionized the lighting industry, some of their characteristics have inhibited the commercialization of their full potential. For example, the emission spectrum of a LED typically exhibits a single, rather narrow peak at a wavelength (the peak wavelength) that is determined by the LED's composition and structure. This is, of course, advantageous in many circumstances, and the art is replete with patents directed to obtaining a precise combination of materials to achieve emission of a specific wavelength of electromagnetic radiation (e.g., blue light). For example, several known devices utilize indium gallium nitride (InGaN) as the active layer (i.e., light emitting layer) in the diode. In InGaN devices, different wavelengths of light are emitted by varying the mole fraction of In in the active layer. Other LEDs are based upon semiconductor materials having specific crystalline structures or active layers that are doped with specific elements in specific concentrations, all of which are done to achieve a specific wavelength of emitted radiation, which may or may not be in the visible spectrum.
Although specificity in light emission is beneficial in many LED applications (e.g., lasers), many otherwise commercially attractive applications of LEDs do not necessarily require the emission specificity inherent in LEDs. Accordingly, researchers are exploring methods to expand the realm of LED applications.
One area for LED expansion is the area of lighting, i.e. artificial (as opposed to daylight) illumination that provides a desired degree of intensity and reproduction of the true color of an object. Intensity refers to the amount of light produced over a given area and is measured in units such as lumens or candelas. Color reproduction is typically measured using the Color Rendering Index (CRI) which is a relative measure of the shift in surface color of an object when lit by a particular lamp. Daylight has the highest CRI (of 100), with incandescent bulbs being relatively close (about 95), and fluorescent lighting being less accurate (70–85). Certain types of specialized lighting have relatively low CRI's (e.g., mercury vapor or sodium, both at about 25).
Because LED's emit at specific wavelengths, they tend to fail (when standing alone) to provide illumination-quality CRI's, regardless of their intensity. Additionally, the nature of light and color are such that some colors (including “white” light) can only be reproduced by a combination of two or more “primary” colors, and thus cannot be reproduced by an individual, unpackaged semiconductor diode.
Nonetheless, because about one-third of the electricity generated in the United States each year goes to lighting, the efficiency of LED's provides a theoretically desirable option. Researchers have known for many years that incandescent light bulbs are very energy inefficient light sources: about ninety (90) percent of the electricity they use is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent bulbs (by a factor of about 4) but are still quite inefficient as compared to solid state LEDs. Furthermore, as compared to the normal lifetimes of semiconductor-based devices, incandescent bulbs have relative short lifetimes; i.e. about 750–1000 hours. In comparison, the lifetime of LEDs can often be measured in decades. Fluorescent bulbs have longer lifetimes (10–20,000 hours), but less favorable CRI's. Accordingly, researchers seek avenues to replace incandescent and fluorescent bulbs with more efficient LEDs.
One way in which the realm of LED applications has been expanded into areas previously dominated by incandescent and fluorescent bulbs is through the development and use of “white” LEDs. Because light that is perceived as white is in reality a blend of two or more colors, “white” photons do not exist and LED's, standing alone, do not produce white photons or white light. Thus, in general terms a white LED is either an LED pixel formed of respective red, green and blue LED's, or a LED that includes a luminescent material (phosphor) that emits white light (or a color that blends to form white light) in response to the particular wavelength emitted by the uncoated LED.
The luminescent materials are often mixed with the package material. Phosphors are common luminescent materials that are mixed with packaging materials. A phosphor is a luminescent material that emits a responsive radiation (e.g., visible light) when excited by a source of exciting radiation. In many instances the responsive radiation has a different—and for illumination purposes, more favorable—wavelength (frequency) than the exciting radiation. Phosphors are used, for example, in cathode ray tubes (of which a television tube is a typical example). A phosphor layer is applied to the wall of a cathode ray tube. An electron beam hits and excites the phosphor layer, causing the phosphor particles to emit light. Other examples of luminescent materials include fluorescent light bulbs, day glow tapes and inks which glow in the visible spectrum upon illumination with ultraviolet light.
In many instances phosphors emit light along a broader spectrum than the source of exciting radiation and at longer wavelengths. For example, some white LEDs are based upon LEDs having active layers that emit blue light. These blue-emitting LEDs include a phosphor, for example (but not limited to) a coating of a thin layer of a transparent material containing a phosphor. The phosphor-containing material can also be referred to as the, “conversion medium.” When the blue light passes through the phosphor-containing material a portion of the blue light excites the phosphor which in turn emits yellow light (yellow light has a longer wavelength than blue light). This yellow light mixes with the remaining blue light from the active layer to create a bright white light. Subtle variations in the phosphor coating vary the tint of the white light from a bluish-white to a yellowish-white. Other phosphors may be combined with other LEDs to achieve different tones, colors or effects.
The blending of primary colors to produce combinations of non-primary colors is generally well understood in this and other arts. In general, the CIE Chromaticity Diagram (an international standard for primary colors established in 1931) provides a useful reference for defining colors as a weighted sum of three defined primary colors.
Currently, the inclusion of luminescent materials in LED based devices is accomplished by adding the materials to the plastic encapsulant material discussed above, for example by a blending or coating process. Accordingly, the packaging step is critical for consistency in the color characteristics and quality of the finished LED.
Using phosphors as an example, if the conversion medium is too thick or the phosphor content in the layer is too great “self-absorption” may occur. Self-absorption occurs when light emissions within the packaging layer stay within the packaging layer to excite other phosphor particles and eventually are absorbed back into the LED structure or are otherwise prevented from exiting the device, thus reducing performance (intensity) and efficiency. Additionally, the particle size of phosphors can become an issue by causing unwanted scattering of both the excitation source (the LED light) and the light generated by the phosphor.
The increased use of gallium nitride and other wide-bandgap semiconductors in LEDs that can emit photons in the ultraviolet (UV) portion of the electromagnetic spectrum, presents new obstacles to packaging because ultraviolet light tends to degrade many of the polymers typically used to package LED's. Furthermore, the higher power (GaN) devices currently entering the market require packaging techniques capable of withstanding the higher power output. For example, the radiation flux from some of the latest LEDs is a multiple or even an order of magnitude greater than that of natural sunlight.
Accordingly, there is a need for a packaging technique that reduces or eliminates the self-absorption and light scattering problems found in traditional luminescent technologies and enhances the light emissions from a LED. Similarly, there is a need for new packaging materials that reduce or eliminates the degradation issues inherent to polymer packaging materials.
In accordance with the background discussion, an object of the invention is to provide a light emitting device comprising a packaging material that offers improved resistance to degradation. A further object of the invention is to provide a light emitting device that reduces or eliminates the difficulties associated with known light emitting devices such as self-absorption and light scattering. Another object of the invention is to provide a method for forming a light emitting device that accomplishes the above objectives.